Capacitive deionization (CDI) is a rapidly emerging desalination technology that promises to deliver clean water while storing energy in the electrical double layer (EDL) near a charged surface in a capacitive format. Whereas most research in this subject area has been devoted to using CDI for removing salts, little attention has been paid to the energy storage aspect of the technology. However, it is energy storage that would allow this technology to compete with other desalination processes if this energy could be stored and reused efficiently. This requires that the operational aspects of CDI be optimized with respect to energy used both during the removal of ions as well as during the regeneration cycle. This translates into the fact that currents applied during deionization (charging the EDL) will be different from those used in regeneration (discharge). This paper provides a mechanistic analysis of CDI in terms of energy consumption and energy efficiencies during the charging and discharging of the system under several scenarios. In a previous study, we proposed an operational buffer mode in which an effective separation of deionization and regeneration steps would allow one to better define the energy balance of this CDI process. This paper reports on using this concept, for optimizing energy efficiency, as well as to improve upon the electro-adsorption of ions and system lifetime. Results obtained indicate that real-world operational modes of running CDI systems promote the development of new and unexpected behavior not previously found, mainly associated with the inhomogeneous distribution of ions across the structure of the electrodes.

Chemical functionalization of nanocarbons is an important strategy to produce electrochemical systems with higher energy/power density by generating surface functional groups with additional faradaic contribution, by increasing their surface area and correspondent capacitive contribution and by improving compatibility with aqueous electrolytes and other active materials, such as pseudocapacitive metal-oxides. Here we present an electrochemical method to simultaneously swell and functionalize large electrodes consisting of fabrics of macroscopic fibers of carbon nanotubes that renders the material hydrophilic and produces a substantial increase of specific capacitance and energy density in aqueous electrolytes. Through in-depth characterization of the carbon nanotube fibres (CNTF) by Raman spectroscopy, transmission electron microscopy, X-ray photoelectrocn spectroscopy (XPS) and small-angle X-ray scattering (SAXS) we identify various contributions to such improvements, including surface oxidation, tubular unzipping, debundling and inter-bundle swelling. Changes in hydrophilicity of functionalized CNTF are determined by analyzing the dynamics of spreading of polar and nonpolar liquids in the electrode. The extracted contact angles and polar and dispersive surface energy components for different treatment conditions are in agreement with changes in dipole-moment obtained by XPS. Finally, functionalized CNTF electrodes were employed in current collector-free solid flexible supercapacitors, which show enhanced electrochemical properties compared to as-produced hydrophobic ones. 1. INTRODUCTION The superlative properties of nanocarbons continue to fuel the interest in macroscopic architectures that efficiently exploit their "molecular" properties. The exceptional stiffness, charge mobility and electrochemical stability of CNTs, for example, makes them attractive for applications ranging from lightweight composites, to electrochemical charge storage/transfer processes to biomedical applications. 123 In this quest, macroscopic CNT fibres, yarns and fabrics have emerged as attractive systems in which the CNTs associate in long coherent bundle domains that favor inter-tube charge and stress transfer, while also leaving large mesoporous gaps between bundles and thus giving rise to a large porosity. Such structure leads to an unusual combination of bulk mechanical toughness, electrical conductivity and electrochemical stability above that of many metals, combined with a large specific surface area above 250 m 2 /g. These property envelop is partly the reason of the increasing use of CNT fibres as electrodes/current collectors in multiple energy storage, transfer and conversion devices, including those requiring light weight and augmented mechanical properties 4 .

The pore structure and surface area of CNT-based electrodes is analysed by SAXS/WAXS, including under eletrochemical measurements.

A novel current collector-free electrode for capacitive deionization to treat brackish water, with low energy consumption and exceptional desalination properties directly linked to the nanoparticle metal oxide/carbon nanotube fibre network morphology.

In order for capacitive deionization (CDI) as a water treatment technology to achieve commercial success, substantial improvements in the operational aspects of the system should be improved in order to efficiently recover the energy stored during the deionization step. In the present work, to increase the energy efficiency of the adsorption-desorption processes, we propose a new operational procedure that utilizes a concentrated brine stream as a washing solution during regeneration. Using this approach, we demonstrate that by replacing the electrolyte during regeneration for a solution with higher conductivity, it is possible to substantially increase round-trip energy efficiency. This procedure was experimentally verified in a flow cell reactor using a pair of carbon electrodes (10(2) cm geometric area) and NaCl solutions having concentrations between 50 and 350 mmol·L(-1). According to experimental data, this new operational mode allows for a better utilization of the three-dimensional structure of the porous material. This increases the energetic efficiency of the global CDI process to above 80% when deionization/regeneration currents ratio are optimized for brackish water treatment.

The ever-increasing amount of batteries used in today's society has led to an increase in the demand of lithium in the last few decades. While mining resources of this element have been steadily exploited and are rapidly depleting, water resources constitute an interesting reservoir just out of reach of current technologies. Several techniques are being explored and novel materials engineered. While evaporation is very time-consuming and has large footprints, ion sieves and supramolecular systems can be suitably tailored and even integrated into membrane and electrochemical techniques. This review gives a comprehensive overview of the available solutions to recover lithium from water resources both by passive and electrically enhanced techniques. Accordingly, this work aims to provide in a single document a rational comparison of outstanding strategies to remove lithium from aqueous sources. To this end, practical figures of merit of both main groups of techniques are provided. An absence of a common experimental protocol and the resulting variability of data and experimental methods are identified. The need for a shared methodology and a common agreement to report performance metrics are underlined.

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Capacitive deionization (CDI) is an emerging desalination technology based on the same charge storage principles as in electrical double-layer supercapacitors (EDLC's). In this study, electrodes of differing thicknesses were tested using constant current (CC-CDI) and constant voltage (CV-CDI) operational modes in order to study the best way of sequestering the highest amount of ions under different salt concentration scenarios. CV-CDI was used to calculate electrode time constants (RC) and thereby determine a suitable current density for desalinating the solution. Results showed that the voltage pulse produced a fast but heterogeneous layer of ion adsorption, presumably, on the most accessible part of the electrode surface. Thus, volumetric specific capacitance under CV-CDI mode might vary from 17-24 F cm −3 and 14-20 F cm −3 for 50 μm and 180 μm electrodes, respectively. Nevertheless, results demonstrated that, under the constant current mode, it is possible to increase charge storage by 30% for a CDI cell consisting of thin electrodes and as much as 80% for thicker electrodes simply by controlling current density and, therefore, the rate capability. Moreover, the analysis of the charge efficiency indicated that a proper selection of the current density can result in efficiencies above 80% regardless the salt concentration scenario.